1. Field of the Invention:
This invention relates to time interval control device, and more particularly to a time interval control device suitable for use in exposure control of a photographic camera and which operates with a very high accuracy such that the exposure time error is within a one-eighth step.
2. Description of the Prior Art:
There is known an exposure control device of the type for controlling the shutter speed of a camera in accordance with the level of brightness of an object being photographed, wherein information representative of the object brightness level is converted by an analog-to-digital converter to a digital code which, after once being memorized, is digitally read out by a digital circuit to control the period of actuation of the shutter, as, for example, disclosed in U.S. Pat. No. 3,824,608.
The conventional type exposure control device, however, controls exposure time in one-step progression, the exposure time error amounting to as large as .+-.1/2 step, and accordingly has a drawback that the period of actuation of the shutter can not be controlled with high accuracy. The term "one step" herein used refers to a particular unit of exposure time expressed by APEX value. (APEX: Additive System of Photographic Exposure). The relationship between APEX values and corresponding time intervals in seconds is shown in FIG. 1. For example, APEX value "0" corresponds to one second, APEX value "1" to 1/2 second, and APEX value "1/8" to 0.917 second.